Free-space optical communication dual-fiber ferrule

ABSTRACT

An optical communication terminal is configured to operate in two different complementary modes of full duplex communication. In one mode, the terminal transmits light having a first wavelength and receives light having a second wavelength along a common free space optical path. In the other mode, the terminal transmits light having the second wavelength and receives light having the first wavelength. The terminal includes a steering mirror that directs light to and from a dichroic element that creates different optical paths depending on wavelength, and also includes spatially separated emitters and detectors for the two wavelengths. A first complementary emitter/detector pair is used in one mode, and a second pair is used for the other mode. The system also includes at least two ferrules. Each ferrule operates with a single emitter/detector pair. The ferrules are designed to operate interchangeably with either emitter/detector pair.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to and is a continuation in partof U.S. patent application Ser. No. 14/189,582, filed Feb. 25, 2014,titled “Optical Communication Terminal,” the entire contents of which ishereby incorporated by reference.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Computing devices such as personal computers, laptop computers, tabletcomputers, cellular phones, and countless types of Internet-capabledevices are increasingly prevalent in numerous aspects of modern life.As such, the demand for data connectivity via the Internet, cellulardata networks, and other such networks, is growing. However, there aremany areas of the world where data connectivity is still unavailable, orif available, is unreliable and/or costly.

Free space optical communication links can be formed between respectivecommunication terminals that send and receive modulated laser light. Forexample, a first terminal may generate laser light modulated accordingto output data and transmit the laser light to a second terminal wherethe laser light is detected and demodulated to recover the data.Similarly, the second terminal may generate laser light modulatedaccording to data and transmit laser light to the first terminal wherethe laser light is detected and demodulated.

To support full duplex communication in which data can be sent andreceived simultaneously using a single terminal, configurations maytransmit data using a first wavelength and receive data using a secondwavelength. The communication terminal can then include a dichroic beamsplitter in the optical path from the primary aperture of the terminal.The dichroic beam splitter can allow one of the wavelengths to passthrough and reflect the other to thereby separate the lightsent/received at the two different wavelengths while sharing a singleprimary aperture. For instance, a first terminal can be configured totransmit at wavelength 1 and receive at wavelength 2. A dichroic beamsplitter may pass wavelength 1 and reflect wavelength 2. A laser lightsource configured to emit at wavelength 1 can be situated to emit lightthat passes through the dichroic beam splitter, toward the primaryaperture. A photo detector configured to detect at wavelength 2 can besituated to receive light that is reflected by the dichroic beamsplitter after being received via the primary aperture. Similarly, asecond communication terminal can be configured to transmit atwavelength 2 and receive at wavelength 1. A suitable laser light sourceand photo detector can be arranged with respect to a dichroic beamsplitter to direct light to/from a single primary aperture of the secondterminal. As such, the first and second communication terminals cancommunicate data in either direction simultaneously by sending andreceiving modulated laser beams at the two different wavelengths.

To ensure alignment between spatially separated terminals, each terminalgenerally incorporates one or more adjustable beam steering mirrors thatdirect laser light to and from the respective transmit/receive aperturesto the various laser light sources and photo detectors in each terminal.Adjusting orientations of the beam steering mirror(s) may then adjustthe positions of various focal points in the optical path(s) couplingvarious laser light source(s) and photo detector(s) to the primaryaperture. A feedback system may also be used to detect an angle ofarrival of incoming laser light (e.g., from another terminal), and usethe determined angle as feedback to direct transmitted laser light(e.g., to the other terminal). The optical path between the primaryaperture and the various laser light source(s) and/or photo detector(s)may additionally include a variety of filters, mirrors, lenses,apertures, and other optical transmission components as necessary.

SUMMARY

Disclosed herein is a ferrule used to connect two fiber optic cables toan optical communication terminal. One of the two fiber optic cables maybe a cable that provides light for transmission by the opticalcommunication terminal. The other one of the two fiber optic cables maybe a cable that communications light received by the opticalcommunication terminal for processing. The optical communicationterminal may operate with two different frequencies of light. Each ofthe frequencies may have its own respective output port on thecommunication terminal. And, each respective output port may beconfigured to connect to a ferrule. The ferrule connector may beinterchangeable between the two output ports. That is, the location ofthe fiber optic cables within a ferrule may be the same regardless ofwhich port to which the ferrule connects.

Example embodiments relate to an optical communication terminal. Theoptical communication terminal includes a beam splitter configured totransmit light of a first wavelength and reflect light of a secondwavelength. The optical communication terminal further includes at leasttwo ferrules—at least a first ferrule and a second ferrule. Each ferruleincludes a connector, a transmission fiber, and a reception fiber. Thefirst ferrule is configured to couple light transmittable by the beamsplitter to at least one fiber and the second ferrule is configured tocouple light reflectable by the beam splitter to at least one fiber. Theoptical communication terminal also includes a steering mirror the ispositionable in at least a first orientation and a second orientation.The steering mirror and the beam splitter may be arranged such that,while the steering mirror has a first orientation, (i) light of thefirst wavelength that is emitted from the transmission fiber of a firstferrule is directed for transmission toward a remote terminal, and (ii)light of the second wavelength that is received from the remote terminalis directed toward the reception fiber of a second ferrule. The steeringmirror and the beam splitter may be further arranged such that, whilethe steering mirror has a second orientation, (i) light of the secondwavelength that is emitted from the transmission fiber of the secondferrule is directed for transmission toward a remote terminal, and (ii)light of the first wavelength that is received from the remote terminalis directed toward the reception fiber of the first ferrule.

Some embodiments of the present disclosure provide a high altitudeplatform. The high altitude platform can include an envelope, a payloadconfigured to be suspended from the envelope, and an opticalcommunication terminal mounted to the payload. The optical communicationterminal may include (i) a beam splitter configured to transmit light ofa first wavelength and to reflect light of a second wavelength, (ii) atleast two ferrules, a first ferrule and a second ferrule, and (iii) asteering mirror. Each ferrule includes a connector, a transmissionfiber, and a reception fiber. The first ferrule is configured to couplelight transmittable by the beam splitter to at least one fiber and thesecond ferrule is configured to couple light reflectable by the beamsplitter to at least one fiber. The steering mirror may be positionablein at least a first orientation and a second orientation. The steeringmirror and the beam splitter are arranged such that, while the steeringmirror has a first orientation, (i) light of the first wavelength thatis emitted from the transmission fiber of a first ferrule is directedfor transmission toward a remote terminal, and (ii) light of the secondwavelength that is received from the remote terminal is directed towardthe reception fiber of a second ferrule. The steering mirror and thebeam splitter may be further arranged such that, while the steeringmirror has a second orientation, (i) light of the second wavelength thatis emitted from the transmission fiber of the second ferrule is directedfor transmission toward a remote terminal, and (ii) light of the firstwavelength that is received from the remote terminal is directed towardthe reception fiber of the first ferrule.

Some embodiments of the present disclosure provide a method. The methodcan include making a determination to conduct full duplex communicationin one of two modes. Responsive to making the determination toconducting full duplex communication in a first mode, the method mayinclude orienting a steering mirror so as to: (i) direct light of afirst wavelength that is emitted from a transmission fiber of a firstferrule toward a remote terminal, and (ii) direct incident light of asecond wavelength received from the remote terminal toward a receptionfiber of a second ferrule. As part of the first mode, the method mayalso include conducting full duplex communication in a first mode by:(i) emitting light of the first wavelength from transmission fiber ofthe first ferrule, modulated based on output data, and (ii) receivinglight of the second wavelength by the reception fiber of the secondferrule. Responsive to making the determination to conduct full duplexcommunication in a second mode, the method may include orienting thesteering mirror so as to: (i) direct light of the second wavelengthemitted from a transmission fiber of the second ferrule toward theremote terminal, and (ii) direct incident light of the first wavelengthreceived from the remote terminal toward a reception fiber of the firstferrule. As part of the second mode, the method may also includeconducting full duplex communication in the second mode by: (i) emittinglight of the second wavelength from transmission fiber of the secondferrule, modulated based on output data, and (ii) receiving light of thefirst wavelength by the reception fiber of the first ferrule.

These as well as other aspects, advantages, and alternatives, willbecome apparent to those of ordinary skill in the art by reading thefollowing detailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified block diagram illustrating an example balloonnetwork.

FIG. 2 is a block diagram illustrating an example balloon-networkcontrol system.

FIG. 3 is a simplified block diagram illustrating an examplehigh-altitude balloon.

FIG. 4A is a diagram of a network of balloons communicating with oneanother via free space optical links.

FIG. 4B is a simplified block diagram of an optical communicationterminal.

FIG. 4C is a simplified block diagram of an optical communicationterminal aligned for operation in a first mode of full duplexcommunication.

FIG. 4D is a simplified block diagram of an optical communicationterminal aligned for operation in a second mode of full duplexcommunication.

FIG. 5 is a diagram of an example optical communication terminalarranged to operate alternately in two different modes.

FIG. 6 is a diagram of another example optical communication terminal.

FIG. 7 is a diagram of another example optical communication terminal.

FIG. 8A is a diagram of an example optical communication ferrule.

FIG. 8B is a diagram of another example optical communication ferrule.

FIG. 9 is a flowchart of an example process for operating an opticalcommunication terminal.

FIG. 10 illustrates a computer readable medium according to an exampleembodiment.

DETAILED DESCRIPTION

Example methods and systems are described herein. Any example embodimentor feature described herein is not necessarily to be construed aspreferred or advantageous over other embodiments or features. Theexample embodiments described herein are not meant to be limiting. Itwill be readily understood that certain aspects of the disclosed systemsand methods can be arranged and combined in a wide variety of differentconfigurations, all of which are contemplated herein.

1. Overview

Example embodiments relate to an aerial communication network ofballoons, and, in particular, to communication equipment to facilitatewireless communication between balloons and/or between balloons andground-based stations. A network of powered balloons can be situated inthe atmosphere and equipped with telecommunications equipment andconfigured to communicate amongst one another and with terminals on theground, and perhaps with satellites. The balloons can each serve asterminals in an aerial interconnected network capable of communicationwith one another, with ground-based user equipment, and/or withterrestrial data networks (e.g., the Internet and/or cellular datanetworks) so as to provide user equipment with access to such datanetworks via the aerial network.

Balloons can communicate with one another over free space opticalcommunication links. Each balloon can be equipped with lasertransmitters and detectors, and steerable apertures and optical trainscapable of directing laser light emitted from one balloon to a detectoron another. To send data from one balloon to another, the laser lightfrom one balloon can be modulated to encode data, the modulated lightcan be transmitted through the atmosphere, and directed to anotherballoon in the network, the other balloon can then receive the laserlight, and recover the encoded data.

To form a mesh network amongst balloons in the network, each givenballoon in the network may send and receive signals between multipleother balloons. In addition, each communication link may allow forbi-directional communications (e.g., balloon one may send signals toballoon two and balloon two may also send signals to balloon one). Toefficiently utilize the available resources, each communication link maysupport full duplex communications by a first balloon transmitting anoptical beam to a second balloon while simultaneously receiving adistinguishable optical beam from the second balloon (e.g., using onewavelength to carry communication in one direction and anotherwavelength to carry communication in the other direction).

To effectively communicate in such a full duplex mode, one balloon canbe in a first mode in which data is sent at wavelength 1, λ1, (i.e. afirst frequency) and received at wavelength 2, λ2 (i.e. a secondfrequency); and the other balloon can be in a second mode in which datais sent at λ2 and received at λ1. However, as various connections aremade over time between different balloons, a given balloon may need toswitch modes in order to facilitate full duplex communication withanother balloon. That is, a full duplex communication link between twoballoons initially in the first mode (or two in the second mode) may notbe possible unless one of the balloons switches from the first mode tothe second mode (or the second mode to the first mode).

The disclosure herein provides an arrangement for an opticalcommunication terminal in which switching between the first mode and thesecond mode is accomplished by changing an alignment of a steeringmirror. An example communication terminal includes: a first ferruleconfigured with a light source fiber configured to emit laser lighthaving a first wavelength and a light reception fiber configured toreceive light having the first wavelength, and a second ferruleconfigured with a light source fiber configured to emit laser lighthaving a second wavelength and a light reception fiber configured toreceive light having the second wavelength.

In some examples, a fiber optic ferrule is a structure attached at thetip of optical fiber. The ferrule allows the handling and processing ofthe fiber tip and often becomes part of fiber optic connectors. Ferulesare commonly made of ceramic, glass or metals.

A steering mirror directs laser light to and from a primary aperture anda dichroic beam splitter. The dichroic beam splitter substantiallytransmits laser light having the first wavelength and substantiallyreflects laser light having the second wavelength to thereby separatethe optical paths of the light having the two different wavelengths. Thesteering mirror can be adjusted from a first approximate orientation, inwhich the optical terminal is configured to operate in the first mode,to a second approximate orientation, in which the optical terminal isconfigured to operate in the second mode. In particular, in the firstapproximate orientation, laser light having the first wavelength emittedfrom the first emission location is directed to the primary aperture andlaser light having the second wavelength received via the primaryaperture is directed to the second detection location. And in the secondapproximate orientation, laser light having the second wavelengthemitted from the second emission location is directed to the primaryaperture and laser light having the first wavelength received via theprimary aperture is directed to the first detection location.

An orientation feedback sensor can be used to estimate an orientation ofthe steering mirror, and then adjustments can be made based on theestimated orientation. The orientation feedback sensor can include abeam splitter that diverts some light reflected by the steering mirror(from the primary aperture) to an array of photo-sensitive elements,which can be used to measure the intensity of light illuminating thearray. A controller can be used to identify a centroid position of thelight illuminating the array, and adjust the steering mirror to causethe centroid position to move toward a target location corresponding tothe first or second orientations, depending on the mode of operation.The first and second approximate orientations, and corresponding targetlocations on the photo-sensitive array may be determined in part duringa calibration routine, for example. Because the steering mirror is usedto direct both received light from the primary aperture, and transmittedlight from the laser light source(s), the laser light transmitted fromthe terminal can be aligned toward the source of the receive light(e.g., a primary aperture of the other terminal).

In another example, the alignment laser light may be laser light with athird wavelength different from the wavelengths used to carrycommunications. In that case, the alignment laser light may be directedtoward the photo-sensitive array using a dichroic beam splitter thatreflects the alignment laser light while transmitting the data carryinglaser wavelengths. Moreover, such a dichroic beam splitter may reflectthe two data-carrying laser wavelengths and transmit the alignment laserwavelength.

To operate in the first mode (transmit λ1, receive λ2), the terminal canactivate the laser light source that emits modulated laser light at λ1and the photo detector that detects laser light at λ2, and orient thesteering mirror in the first orientation. And to operate in the secondmode (transmit λ2, receive λ1), the terminal can activate the laserlight source that emits modulated laser light at λ2 and the photodetector that detects laser light at λ1, and orient the steering mirrorin the second orientation. Switching between modes can be performed byadjusting the orientation of the steering mirror andactivating/deactivating the appropriate lasers and detectors.

Each of these specific methods and systems are contemplated herein, andseveral example embodiments are described below.

2. Example Systems

FIG. 1 is a simplified block diagram illustrating a balloon network 100,according to an example embodiment. As shown, balloon network 100includes balloons 102A to 102F, which are configured to communicate withone another via free-space optical links 104 (e.g., by sending andreceiving optical radiation encoded with data). Moreover, while referredto as “optical,” communication on the optical links 104 may be carriedout with radiation at a range of wavelengths including radiation outsidethe visible spectrum, such as infrared radiation, ultraviolet radiation,etc. Balloons 102A to 102F could additionally or alternatively beconfigured to communicate with one another via radio frequency (RF)links 114 (e.g., by sending and receiving radio frequency radiationencoded with data). Balloons 102A to 102F may collectively function as amesh network for packet-data communications. Further, at least someballoons (e.g., 102A and 102B) may be configured for RF communicationswith a ground-based station 106 via respective RF links 108. Further,some balloons, such as balloon 102F, could be configured to communicatevia optical link 110 with a suitably equipped ground-based station 112.

In an example embodiment, balloons 102A to 102F are high-altitudeballoons, which are deployed in the stratosphere. At moderate latitudes,the stratosphere includes altitudes between approximately 10 kilometers(km) and 50 km above the surface of the Earth. At the poles, thestratosphere starts at an altitude of approximately 8 km. In an exampleembodiment, high-altitude balloons may be generally configured tooperate in an altitude range within the stratosphere that has relativelylow wind speed (e.g., between 8 and 32 kilometers per hour (kph)).

More specifically, in a high-altitude balloon network, balloons 102A to102F may generally be configured to operate at altitudes between 18 kmand 25 km (although other altitudes are possible). This altitude rangemay be advantageous for several reasons. In particular, this altituderegion of the stratosphere generally has relatively desirableatmospheric conditions with low wind speeds (e.g., winds between 8 and32 kph) and relatively little turbulence. Further, while winds betweenaltitudes of 18 km and 25 km may vary with latitude and by season, thevariations can be modeled with reasonably accuracy and thereby allow forpredicting and compensating for such variations. Additionally, altitudesabove 18 km are typically above the maximum altitude designated forcommercial air traffic.

To transmit data to another balloon, a given balloon 102A to 102F may beconfigured to transmit an optical signal via an optical link 104. In anexample embodiment, a given balloon 102A to 102F may use one or morehigh-power light-emitting diodes (LEDs) to transmit an optical signal.Alternatively, some or all of balloons 102A to 102F may include lasersystems for free-space optical communications over optical links 104.Other types of free-space optical communication are possible. Further,in order to receive an optical signal from another balloon via anoptical link 104, a given balloon 102A to 102F may include one or moreoptical detectors, such as avalanche photo diodes.

In a further aspect, balloons 102A to 102F may utilize one or more ofvarious different RF air-interface protocols for communication withground-based stations 106 and 112 via respective RF links 108. Forinstance, some or all of balloons 102A to 102F may be configured tocommunicate with ground-based stations 106 and 112 using protocolsdescribed in IEEE 802.11 (including any of the IEEE 802.11 revisions),various cellular protocols such as GSM, CDMA, UMTS, EV-DO, WiMAX, and/orLTE, and/or one or more propriety protocols developed for balloon-groundRF communication, among other possibilities.

In a further aspect, there may be scenarios where RF links 108 do notprovide a desired link capacity for balloon-to-ground communications.For instance, increased capacity may be desirable to provide backhaullinks from a ground-based gateway, and in other scenarios as well.Accordingly, an example network may also include one or more downlinkballoons, which could provide a high-capacity air-ground link to connectthe balloon network 100 to ground-based network elements.

For example, in balloon network 100, balloon 102F is configured as adownlink balloon. Like other balloons in an example network, thedownlink balloon 102F may be operable for optical communication withother balloons via optical links 104. However, the downlink balloon 102Fmay also be configured for free-space optical communication with aground-based station 112 via an optical link 110. Optical link 110 maytherefore serve as a high-capacity link (as compared to an RF link 108)between the balloon network 100 and the ground-based station 112.

Note that in some implementations, the downlink balloon 102F mayadditionally be operable for RF communication with ground-based stations106. In other cases, the downlink balloon 102F may only use an opticallink for balloon-to-ground communications. Further, while thearrangement shown in FIG. 1 includes just one downlink balloon 102F, anexample balloon network can also include multiple downlink balloons. Onthe other hand, a balloon network can also be implemented without anydownlink balloons.

In other implementations, a downlink balloon may be equipped with aspecialized, high-bandwidth RF communication system forballoon-to-ground communications, instead of, or in addition to, afree-space optical communication system. The high-bandwidth RFcommunication system may take the form of an ultra-wideband system,which may provide an RF link with substantially the same capacity as oneof the optical links 104. Other forms are also possible.

Ground-based stations, such as ground-based stations 106 and/or 112, maytake various forms. Generally, a ground-based station may includecomponents such as transceivers, transmitters, and/or receivers forwireless communication via RF links and/or optical links withcorresponding transceivers situated on balloons in the balloon network100. Further, a ground-based station may use various air-interfaceprotocols to communicate with balloons 102A to 102F over an RF link 108.As such, ground-based stations 106 and 112 may be configured as anaccess point via which various devices can connect to balloon network100. Ground-based stations 106 and 112 may have other configurationsand/or serve other purposes without departing from the scope of thepresent disclosure.

In a further aspect, some or all of balloons 102A to 102F could beadditionally or alternatively configured to establish a communicationlink with space-based satellites. In some embodiments, a balloon maycommunicate with a satellite via an optical link. However, other typesof satellite communications are possible.

Further, some ground-based stations, such as ground-based stations 106and 112, may be configured as gateways between balloon network 100 andone or more other networks. Such ground-based stations 106 and 112 maythus serve as an interface between the balloon network and the Internet,a cellular service provider's network, and/or other types of networksfor communicating information. Variations on this configuration andother configurations of ground-based stations 106 and 112 are alsopossible.

2a) Mesh Network Functionality

As noted, balloons 102A to 102F may collectively function as a meshnetwork. More specifically, since balloons 102A to 102F may communicatewith one another using free-space optical links, the balloons maycollectively function as a free-space optical mesh network.

In a mesh-network configuration, each balloon 102A to 102F may functionas a node of the mesh network, which is operable to receive datadirected to it and to route data to other balloons. As such, data may berouted from a source balloon to a destination balloon by determining anappropriate sequence of optical links between the source balloon and thedestination balloon. These optical links may be collectively referred toas a “lightpath” for the connection between the source and destinationballoons. Further, each of the optical links may be referred to as a“hop” on the lightpath. Each intermediate balloon (i.e., hop) along aparticular lightpath may act as a repeater station to first detect theincoming communication via received optical signals and then repeat thecommunication by emitting a corresponding optical signal to be receivedby the next balloon on the particular lightpath. Additionally oralternatively, a particular intermediate balloon may merely directincident signals toward the next balloon, such as by reflecting theincident optical signals to propagate toward the next balloon.

To operate as a mesh network, balloons 102A to 102F may employ variousrouting techniques and self-healing algorithms. In some embodiments, theballoon network 100 may employ adaptive or dynamic routing, where alightpath between a source and destination balloon is determined andset-up when the connection is needed, and released at a later time.Further, when adaptive routing is used, the lightpath may be determineddynamically depending upon the current state, past state, and/orpredicted state of the balloon network 100.

In addition, the network topology may change as the balloons 102A to102F move relative to one another and/or relative to the ground.Accordingly, an example balloon network 100 may apply a mesh protocol toupdate the state of the network as the topology of the network changes.For example, to address the mobility of the balloons 102A to 102F,balloon network 100 may employ and/or adapt various techniques that areemployed in mobile ad hoc networks (MANETs). Other examples are possibleas well.

In some implementations, the balloon network 100 may be configured as atransparent mesh network. More specifically, in a transparent meshnetwork configuration, the balloons may include components for physicalswitching that are entirely optical, without any electrical componentsinvolved in the routing of optical signals. Thus, in a transparentconfiguration with optical switching, signals can travel through amulti-hop lightpath that is entirely optical.

In other implementations, the balloon network 100 may implement afree-space optical mesh network that is opaque. In an opaqueconfiguration, some or all balloons 102A to 102F may implementoptical-electrical-optical (OEO) switching. For example, some or allballoons may include optical cross-connects (OXCs) for OEO conversion ofoptical signals. Other opaque configurations are also possible.Additionally, network configurations are possible that include routingpaths with both transparent and opaque sections.

In a further aspect, balloons in the balloon network 100 may implementwavelength division multiplexing (WDM), which may be used to increaselink capacity. When WDM is implemented with transparent switching, itmay be necessary to assign the same wavelength for all optical links ona given lightpath. Lightpaths in transparent balloon networks aretherefore said to be subject to a “wavelength continuity constraint,”because each hop in a particular lightpath may be required to use thesame wavelength.

An opaque configuration, on the other hand, may avoid such a wavelengthcontinuity constraint. In particular, balloons in an opaque balloonnetwork may include OEO switching systems operable for wavelengthconversions along a given lightpath. As a result, balloons can convertthe wavelength of an optical signal at one or more hops along aparticular lightpath.

Moreover, some example mesh networks may utilize both optical links andRF links. For example, communication pathway through the mesh networkmay involve one or more hops on optical links, and one or more hops onRF links. RF links may be used, for instance, between relativelyproximate balloons in the network.

2b) Control of Balloons in a Balloon Network

In some embodiments, mesh networking and/or other control functions maybe centralized. For example, FIG. 2 is a block diagram illustrating aballoon-network control system, according to an example embodiment. Inparticular, FIG. 2 shows a distributed control system, which includes acentral control system 200 and a number of regional control-systems 202Ato 202B. Such a control system may be configured to coordinate certainfunctionality for balloon network 204, and as such, may be configured tocontrol and/or coordinate certain functions for balloons 206A to 206I.

In the illustrated embodiment, central control system 200 may beconfigured to communicate with balloons 206A to 206I via a number ofregional control systems 202A to 202C. These regional control systems202A to 202C may be configured to receive communications and/oraggregate data from balloons in the respective geographic areas thatthey cover, and to relay the communications and/or data to centralcontrol system 200. Further, regional control systems 202A to 202C maybe configured to route communications from central control system 200 tothe balloons in their respective geographic areas. For instance, asshown in FIG. 2, regional control system 202A may relay communicationsand/or data between balloons 206A to 206C and central control system200, regional control system 202B may relay communications and/or databetween balloons 206D to 206F and central control system 200, andregional control system 202C may relay communications and/or databetween balloons 206G to 206I and central control system 200.

In order to facilitate communications between the central control system200 and balloons 206A to 206I, certain balloons may be configured asdownlink balloons, which are operable to communicate with regionalcontrol systems 202A to 202C. Accordingly, each regional control system202A to 202C may be configured to communicate with the downlink balloonor balloons in the respective geographic area it covers. For example, inthe illustrated embodiment, balloons 206A, 206F, and 206I are configuredas downlink balloons. As such, regional control systems 202A to 202C mayrespectively communicate with balloons 206A, 206F, and 206I via opticallinks 208, 210, and 212, respectively.

In the illustrated configuration, only some of balloons 206A to 206I areconfigured as downlink balloons. The balloons 206A, 206F, and 206I thatare configured as downlink balloons may relay communications fromcentral control system 200 to other balloons in the balloon network,such as balloons 206B to 206E, 206G, and 206H. However, it should beunderstood that in some implementations, it is possible that allballoons may function as downlink balloons. Further, while FIG. 2 showsmultiple balloons configured as downlink balloons, it is also possiblefor a balloon network to include only one downlink balloon. Moreover, aballoon network may additionally or alternatively include satellite linkballoons which communicate with a central control system and/or datatransport networks via connection with a satellite network, which mayinvolve, for example, communication over free space optical links withsatellites in a communication network orbiting above the balloonnetwork.

The regional control systems 202A to 202C may be particular types ofground-based stations that are configured to communicate with downlinkballoons (e.g., such as ground-based station 112 of FIG. 1). Thus, whilenot shown in FIG. 2, a control system may be implemented in conjunctionwith other types of ground-based stations (e.g., access points,gateways, etc.).

In a centralized control arrangement, such as that shown in FIG. 2, thecentral control system 200 (and possibly regional control systems 202Ato 202C as well) may coordinate certain mesh-networking functions forballoon network 204. For example, balloons 206A to 206I may send thecentral control system 200 certain state information, which the centralcontrol system 200 may utilize to determine the state of balloon network204. The state information from a given balloon may include locationdata, optical-link information (e.g., the identity of other balloonswith which the balloon has established an optical link, the bandwidth ofthe link, wavelength usage and/or availability on a link, etc.), winddata collected by the balloon, and/or other types of information.Accordingly, the central control system 200 may aggregate stateinformation from some or all of the balloons 206A to 206I in order todetermine an overall state of the network 204.

Based in part on the overall state of the network 204, the controlsystem 200 may then be used to coordinate and/or facilitate certainmesh-networking functions, such as determining lightpaths forconnections, for example. The central control system 200 may determine acurrent topology (or spatial distribution of balloons) based on theaggregate state information from some or all of the balloons 206A to206I. The topology may indicate the current optical links that areavailable in the balloon network and/or the wavelength availability onsuch links. The topology may then be sent to some or all of the balloonsso that individual balloons are enabled to select appropriate lightpaths(and possibly backup lightpaths) for communications through the balloonnetwork 204 as needed.

In a further aspect, the central control system 200 (and possiblyregional control systems 202A to 202C as well) may also coordinatecertain positioning functions for balloon network 204 to achieve adesired spatial distribution of balloons. For example, the centralcontrol system 200 may input state information that is received fromballoons 206A to 206I to an energy function, which may effectivelycompare the current topology of the network to a desired topology, andprovide a vector indicating a direction of movement (if any) for eachballoon, such that the balloons can move towards the desired topology.Further, the central control system 200 may use altitudinal wind data todetermine respective altitude adjustments that may be initiated toachieve the movement towards the desired topology. The central controlsystem 200 may provide and/or support other station-keeping functions aswell.

FIG. 2 shows a distributed arrangement that provides centralizedcontrol, with regional control systems 202A to 202C coordinatingcommunications between a central control system 200 and a balloonnetwork 204. Such an arrangement may be useful to provide centralizedcontrol for a balloon network that covers a large geographic area. Insome embodiments, a distributed arrangement may even support a globalballoon network that provides coverage everywhere on earth. Of course, adistributed-control arrangement may be useful in other scenarios aswell.

Further, it should be understood that other control-system arrangementsare also possible. For instance, some implementations may involve acentralized control system with additional layers (e.g., sub-regionsystems within the regional control systems, and so on). Alternatively,control functions may be provided by a single, centralized, controlsystem, which communicates directly with one or more downlink balloons.

In some embodiments, control and coordination of a balloon network maybe shared by a ground-based control system and a balloon network tovarying degrees, depending upon the implementation. In fact, in someembodiments, there may be no ground-based control systems. In such anembodiment, all network control and coordination functions may beimplemented by the balloon network itself (e.g., by processing systemssituated on payloads of one or more balloons in the network 204). Forexample, certain balloons may be configured to provide the same orsimilar functions as central control system 200 and/or regional controlsystems 202A to 202C. Other examples are also possible.

Furthermore, control and/or coordination of a balloon network may bede-centralized. For example, each balloon may relay state informationto, and receive state information from, some or all nearby balloons.Further, each balloon may relay state information that it receives froma nearby balloon to some or all nearby balloons. When all balloons doso, each balloon may be able to individually determine the state of thenetwork. Alternatively, certain balloons may be designated to aggregatestate information for a given portion of the network. These balloons maythen coordinate with one another to determine the overall state of thenetwork.

Further, in some aspects, control of a balloon network may be partiallyor entirely localized, such that it is not dependent on the overallstate of the network. For example, individual balloons may implementballoon-positioning functions that only consider nearby balloons. Inparticular, each balloon may determine how to move (and/or whether tomove) based on its own state and the states of nearby balloons. Theballoons may use an optimization routine (e.g., an energy function) todetermine respective absolute and/or relative target positions for each.The respective balloons can then move toward their respective targetpositions, for example, with respect to the nearby balloons, withoutnecessarily considering the desired topology of the network as a whole.However, when each balloon implements such a position determinationroutine, the balloon network as a whole may maintain and/or move towardsthe desired spatial distribution (topology).

2c) Example Balloon Configuration

Various types of balloon systems may be incorporated in an exampleballoon network. As noted above, an example embodiment may utilizehigh-altitude balloons that operate in an altitude range between 18 kmand 25 km. FIG. 3 illustrates a high-altitude balloon 300, according toan example embodiment. As shown, the balloon 300 includes an envelope302, a skirt 304, and a payload 306, which is shown as a block diagram.

The envelope 302 and skirt 304 may take various forms, which may becurrently well-known or yet to be developed. For instance, the envelope302 and/or skirt 304 may be made of metallic and/or polymeric materialsincluding metalized Mylar or BoPet. Additionally or alternatively, someor all of the envelope 302 and/or skirt 304 may be constructed from ahighly-flexible latex material or a rubber material such as chloroprene.Other materials are also possible. The envelope 302 may be filled with agas suitable to allow the balloon 300 to reach desired altitudes in theEarth's atmosphere. Thus, the envelope 302 may be filled with arelatively low-density gas, as compared to atmospheric mixtures ofpredominantly molecular nitrogen and molecular oxygen, to allow theballoon 300 to be buoyant in the Earth's atmosphere and reach desiredaltitudes. Various different gaseous materials with suitable propertiesmay be used, such as helium and/or hydrogen. Other examples of gaseousmaterials (including mixtures) are possible as well.

The payload 306 of balloon 300 may include a computer system 312 havinga processor 313 and on-board data storage, such as memory 314. Thememory 314 may take the form of or include a non-transitorycomputer-readable medium. The non-transitory computer-readable mediummay have instructions stored thereon, which can be accessed and executedby the processor 313 in order to carry out the balloon functionsdescribed herein. Thus, processor 313, in conjunction with instructionsstored in memory 314, and/or other components, may function as acontroller of balloon 300.

The payload 306 of balloon 300 may also include various other types ofequipment and systems to provide a number of different functions. Forexample, payload 306 may include an optical communication system 316,which may transmit optical signals via an ultra-bright LED system and/orlaser system, and which may receive optical signals via anoptical-communication receiver (e.g., a photodiode receiver system).Further, payload 306 may include an RF communication system 318, whichmay transmit and/or receive RF communications via an antenna system.

The payload 306 may also include a power supply 326 to supply power tothe various components of balloon 300. The power supply 326 couldinclude a rechargeable battery or other energy storage devices. Theballoon 300 may include a solar power generation system 327. The solarpower generation system 327 may include solar panels and could be usedto generate power that charges and/or is distributed by the power supply326. In other embodiments, the power supply 326 may additionally oralternatively represent other means for generating and/or supplyingpower.

The payload 306 may additionally include a positioning system 324. Thepositioning system 324 could include, for example, a global positioningsystem (GPS), an inertial navigation system, and/or a star-trackingsystem. The positioning system 324 may additionally or alternativelyinclude various motion sensors (e.g., accelerometers, magnetometers,gyroscopes, and/or compasses). The positioning system 324 mayadditionally or alternatively include one or more video and/or stillcameras, and/or various sensors for capturing environmental dataindicative of the geospatial position of the balloon 300, whichinformation may be used by the computer system 312 to determine thelocation of the balloon 300.

Some or all of the components and systems within payload 306 may beimplemented in a radiosonde or other probe, which may be operable tomeasure environmental parameters, such as pressure, altitude,geographical position (latitude and longitude), temperature, relativehumidity, and/or wind speed and/or wind direction, among otherinformation.

As noted, balloon 300 may include an ultra-bright LED system forfree-space optical communication with other balloons. As such, opticalcommunication system 316 may be configured to generate an optical signalindicative of output data by modulating light from an LED and/or laser.The optical signal can then be transmitted as a highly directional,collimated beam in free space toward a receptive terminal (on anotherballoon), where data can be extracted from the optical signal.Similarly, the optical communication system 316 can also receive anincoming beam of an optical signal from another terminal. The opticalcommunication system 316 can then detect modulation of the receivedoptical signal and extract input data therefrom. The opticalcommunication system 316 may be implemented with mechanical systemsand/or with hardware, firmware, and/or software. Generally, the mannerin which an optical communication system is implemented may vary,depending upon the particular application. The optical communicationsystem 316 and other associated components are described in furtherdetail below.

In a further aspect, balloon 300 may be configured for altitude control.For instance, balloon 300 may include a variable buoyancy system, whichis configured to change the altitude of the balloon 300 by adjusting thevolume and/or density of the gas in the balloon 300. A variable buoyancysystem may take various forms, and may generally be any system that canchange the volume and/or density of gas in the envelope 302.

In an example embodiment, a variable buoyancy system may include abladder 310 that is located inside of envelope 302. The bladder 310could be an elastic chamber configured to hold liquid and/or gas.Alternatively, the bladder 310 need not be inside the envelope 302. Forinstance, the bladder 310 could be a rigid container holding liquefiedand/or gaseous material that is pressurized in excess of the pressureoutside the bladder 310. The buoyancy of the balloon 300 may thereforebe adjusted by changing the density and/or volume of the gas in bladder310. To change the density in bladder 310, balloon 300 may be configuredwith systems and/or mechanisms for heating and/or cooling the gas inbladder 310. Further, to change the volume, balloon 300 may includepumps or other features for adding gas to and/or removing gas frombladder 310. Additionally or alternatively, to change the volume ofbladder 310, balloon 300 may include release valves or other featuresthat are controllable to allow gas to escape from bladder 310. Multiplebladders 310 could be implemented within the scope of this disclosure.For instance, multiple bladders could be used to improve balloonstability.

In an example embodiment, the envelope 302 could be filled with helium,hydrogen or other gaseous material with density less than typicalatmospheric gas (i.e., “lighter-than-air” gasses). The envelope 302could thus have an associated upward buoyancy force based on itsdisplacement. In such an embodiment, air in the bladder 310 could beconsidered a ballast tank that may have an associated downward ballastforce. In another example embodiment, the amount of air in the bladder310 could be changed by pumping air (e.g., with an air compressor) intoand out of the bladder 310. By adjusting the amount of air in thebladder 310, the ballast force may be controlled. In some embodiments,the ballast force may be used, in part, to counteract the buoyancy forceand/or to provide altitude stability.

In other embodiments, the envelope 302 could be substantially rigid andinclude an enclosed volume. Air could be evacuated from envelope 302while the enclosed volume is substantially maintained. In other words,at least a partial vacuum could be created and maintained within theenclosed volume. Thus, the envelope 302 and the enclosed volume couldbecome lighter than air and provide a buoyancy force. In yet otherembodiments, air or another material could be controllably introducedinto the partial vacuum of the enclosed volume in an effort to adjustthe overall buoyancy force and/or to provide altitude control.

In another embodiment, a portion of the envelope 302 could be a firstcolor (e.g., black) and/or formed of a first material different from therest of envelope 302, which may have a second color (e.g., white) and/ora second material. For instance, the first color and/or first materialcould be configured to absorb a relatively larger amount of solar energythan the second color and/or second material. Thus, rotating the balloonsuch that the first material is facing the sun may act to heat theenvelope 302 as well as the gas inside the envelope 302. In this way,the buoyancy force of the envelope 302 may increase. By rotating theballoon such that the second material is facing the sun, the temperatureof gas inside the envelope 302 may decrease. Accordingly, the buoyancyforce may decrease. In this manner, the buoyancy force of the ballooncould be adjusted by changing the temperature/volume of gas inside theenvelope 302 using solar energy. In such embodiments, it is possiblethat a bladder 310 may not be a necessary element of balloon 300. Thus,in various contemplated embodiments, altitude control of balloon 300could be achieved, at least in part, by adjusting the rotation of theballoon with respect to the sun to selectively heat/cool the gas withinthe envelope 302 and thereby adjust the density of such gas.

Further, a balloon 306 may include a navigation system (not shown). Thenavigation system may implement positioning functions to maintainposition within and/or move to a position in accordance with a desiredspatial distribution of balloons (balloon network topology). Inparticular, the navigation system may use altitudinal wind data todetermine altitudinal adjustments that result in the wind carrying theballoon in a desired direction and/or to a desired location. Thealtitude-control system may then make adjustments to the density of theballoon envelope 302 to effect the determined altitudinal adjustmentsand thereby cause the balloon 300 to move laterally to the desireddirection and/or to the desired location. Additionally or alternatively,desired altitudinal adjustments may be computed by a ground-based orsatellite-based control system and communicated to the balloon 300. Inother embodiments, specific balloons in a balloon network may beconfigured to compute altitudinal adjustments for other balloons andtransmit the adjustment commands to those other balloons.

Several example implementations are described herein. It will beunderstood that there are many ways to implement the devices, systems,and methods disclosed herein. Accordingly, the following examples arenot intended to limit the scope of the present disclosure.

3. Full Duplex Optical Communication

FIG. 4A is a diagram of a network 400 of balloons communicating with oneanother via free space optical links. The network 400 includes a firstballoon 402 with multiple optical communication terminals 403 a, 404 b;a second balloon 406 with multiple optical communication terminals 407b, 408 b; and a third balloon 410 with multiple optical communicationterminals 411 a, 412 a. To form a mesh network amongst balloons in thenetwork 400, each given balloon 402, 406, 410 in the network may sendand receive optical signals between one another. Thus, the first balloon402 and second balloon 406 can communicate by exchanging data modulatedoptical signals between respective terminals 403 a and 406 b alonglightpath 414. Lightpath 414 is a free space optical pathway along whichoptical signals propagate between the two balloons 402, 406, andparticularly between the terminals 403 a, 407 b. Similarly, the firstballoon 402 and the third balloon 410 communicate by exchanging datamodulated optical signals between respective terminals 404 b and 412 aalong lightpath 416. And the second balloon 406 and the third balloon410 communicate by exchanging data modulated optical signals betweenrespective terminals 408 b and 411 a along lightpath 418.

Each of the optical communication links in network 400 allow forbi-directional communication using wavelength division to distinguishbetween transmitted signals and received signals. For example, overlightpath 414, balloon 402 may send data to balloon 406 using lighthaving a first frequency and first wavelength, λ1, and balloon 406 mayalso send data to balloon 402 using light having a second frequency anda second wavelength, λ2. As such, data can be transmitted in bothdirections simultaneously over the same lightpath 414, and the twoterminals 403 a, 407 b can employ wavelength-selective optics, such asdichroic beam splitters, filters, etc., to separate the light at λ1 andλ2, and detect the modulation of received signals. The two terminals 403a, 407 b can also include wavelength-specific light sources, such aslaser diodes and/or lasers that are configured to emit data modulatedlight at either from the transmitted signals λ1 or λ2. Thus, the opticalcommunication terminal 403 a is configured to emit light at wavelengthλ1 that is indicative of output data and to simultaneously detect lightat wavelength λ2 that is indicative of input data. In a complementaryfashion, the optical communication terminal 407 b is configured to emitlight at λ2 that is indicative of output data and to simultaneouslydetect light at λ1 that is indicative of input data. In combinationthen, the optical communication terminals 403 a, 407 b form acomplementary pair that allow for bi-directional (full duplex) datacommunication between the balloons 402, 406 over lightpath 414.

To facilitate full duplex communications between two arbitrary balloons,it is therefore generally necessary for the two balloons to have acomplementary pair of terminals (i.e., one that transmits at λ1 andreceives at λ2, and another that transmits at λ2 and receives at λ1).The two complementary modes of such optical communication terminals toenable full duplex communication may be referred to as a “first mode anda “second mode” for convenience in the description herein. In the firstmode, a given terminal transmits light at λ1 and receives light at λ2.In the second mode, a given terminal transmits at λ2 and receives lightat λ1. Thus, the terminal 403 a is a first mode terminal, while theterminal 407 b is a second mode terminal.

Similarly, the communication link between balloons 402 and 410 overlightpath 416 is terminated by terminal 404 b, which transmits at λ2 andreceives light at λ1 (and is therefore a second mode terminal), andterminal 412 a, which transmits at λ1 and receives light at λ2 (and istherefore a first mode terminal). And the communication link betweenballoons 406 and 410 over lightpath 418 is terminated by terminal 408 b,which transmits at λ2 and receives light at λ1 (and is therefore asecond mode terminal), and terminal 411 a, which transmits at λ1 andreceives light at λ2 (and is therefore a first mode terminal). Each ofthe complementary pairs of optical communication terminals thereforeallow each of the balloons 402, 406, 410 to conduct full duplexcommunications between one another.

FIG. 4B is a simplified block diagram of an optical communicationterminal 420. The optical communication terminal 420 may be any of theindividual terminals 403 a, 404 b, 407 b, 408 b, 411 a, 412 a. Theoptical communication terminal 420 includes a steering mirror 430arranged to direct data modulated laser light 428 to and from fixedoptical components 422 along a light path 431 toward another terminal(e.g., on another balloon in network 400). The fixed optical components422 and steering mirror 430 may be mounted to a frame or otherstructural feature so as to substantially fix the relative positions ofthe various optical components 422 and the steering mirror.

The steering mirror 430 may be a reflective surface with an adjustableorientation that is configured to direct incoming (and outgoing) lightfrom (and to) various directions spanning a field of view (the regionlabelled FOV) of the terminal 420. The steering mirror may be rotatablymounted and include adjustable mechanical components (e.g., steppermotors, etc.) that cause the reflective surface to pivot with respect toan axis of rotation. The fixed optical components 422 include one ormore laser light sources 424 for emitting data modulated light and oneor more laser light detectors 426, for detecting received light. Signalsfrom the detector(s) 426 can then be used to extract incoming data inaccordance with the modulation of the received light. A modem may beused to map output data to data modulated laser light and also to mapdata modulated received light to input data.

The fixed optical components 422 direct emitted light from the laserlight source(s) 424 toward the steering mirror 430 so as to transmitdata modulated laser light along direction 431 (e.g., a lightpath towardanother terminal). The optical components 422 are also configured toreceive incoming light along the same optical axis and direct theincoming light to the laser light detector(s) 426. Because the fixedoptical components 422 provide a shared optical pathway for bothincoming and outgoing data modulated light 428, the steering mirror 430can be used to direct emitted light to be transmitted in direction 431and also to direct incoming light from direction 431 to the detector(s)426. Moving the orientation of the steering mirror 430, as shown bydirectional arrow 432, changes the direction of reflected light toenable the optical communication terminal 420 to conduct communicationwith a terminal in another direction within the field of view FOV. Inpractice, the steering mirror 430 may be configured to pivot/rotateabout multiple axes so as to provide a field of view that extends alongboth an azimuthal angle and an elevation angle. For example, the opticalcommunication terminal 420 may be mounted to a payload of a balloon suchthat the data modulated laser light 428 passing in/out of the fixedoptical components 422 is directed vertical (e.g., approximately normalto the surface of the Earth), and the steering mirror 430 is oriented atapproximately 45 degrees to reflect light between balloons at similaraltitudes. From the perspective of the optical terminal 420, adjustingthe tilt angle of the steering mirror can then provide differentelevation angles, and rotating the steering mirror 430 about an axisparallel to the direction of the data modulated laser light 428 canprovide different azimuthal angles.

In addition to the light source(s) 424 and the light detector(s) 426,the optical communication terminal 420 may also include a variety ofoptical elements (e.g., lenses, filters, reflectors, fibers, apertures,etc.) aligned to provide optical pathway and a controller implementedwith one or more hardware, software, and/or firmware implementedmodules. The controller can be configured to encode data intotransmitted laser light, decode data from received laser light, adjustthe orientation of the steering mirror to align transmitted/receivedlight with a particular lightpath, etc. Feedback sensors that indicatethe orientation of the steering mirror 430 may also be included, such aslinear encoders and/or angle of arrival sensors that detect the angle orreceived light via illumination of a photo-sensitive array.

Over time, the configuration of network 400 may be rearranged for avariety of reasons. As the positions of the balloon change relative toone another, new optical communication links may be formed toaccommodate a new configuration. Moreover, the population of balloons inthe network 400 may change over time, as balloons are retired from thenetwork, or move too far away for line-of-sight optical connections,and/or as balloons are added to the network, or enter a region whereline-of-sight optical connections are possible. As noted above,formation of a full duplex optical communication link between twoarbitrary balloons requires that the two terminals be equipped withcomplementary terminals (e.g., one with a first mode terminal andanother with a second mode terminal). To facilitate formation of suchoptical links between arbitrary pairs of balloons, the opticalcommunication terminals can be configured to change between the twocomplementary modes of full duplex communication (e.g., from the firstmode to the second mode and vice versa).

As shown in FIGS. 4C and 4D, the optical terminal 420 can be configuredto change between modes adjusting the orientation of the steering mirror430. The optical terminal 420 can include spatially separated emittersand detectors arranged to be selectively aligned with a given lightpathby the steering mirror, depending on the mode of operation. For example,the optical terminal may include a first emitter/detector pair thattransmits/receives light to/from a common direction while the steeringmirror has a first orientation. The optical terminal may also include asecond emitter/detector pair that transmits/receives light to/from thesame direction while the steering mirror has a second orientation. Thefirst emitter/detector pair can transmit at λ1 and receive at λ2, so theoptical terminal 420 can be configured to operate in the first modewhile the steering mirror 430 has the first orientation; and the secondemitter/detector pair can transmit at λ2 and receive at λ1, so theoptical terminal 420 can be configured to operate in the second modewhile the steering mirror 430 has the second orientation. Thus, theterminal 420 can include a λ1 laser light source 442 and a λ2 detector444, which are used when operating in the first mode, and also a λ2laser light source 450 and a λ1 detector 452, which are used whenoperating in the second mode.

FIG. 4C is a simplified block diagram of the optical communicationterminal 420 aligned for operation in a first mode of full duplexcommunication (e.g., first mode). In the first mode, the opticalcommunication terminal 420 transmits at λ1 and receives at λ2, and thesteering mirror 430 directs the data modulated light 428 to/from fixedoptical components in direction 431 (toward another terminal). Theoptical components include a dichroic beam splitter 440, whichsubstantially reflects λ1 light and substantially transmits λ2 light.The dichroic beam splitter 440 thereby provides wavelength-dependentoptical pathways through the optical terminal 420 and thereby separatetransmitted λ1 light 446 from received λ2 light 448. The dichroic beamsplitter 440 thereby allows the λ1 light 446 to be emitted from aspatially distinct location, within the optical terminal, than alocation where the received λ2 light 448 is detected, yet the λ1 lightand λ2 light (428) propagate along a common optical path (although inopposite directions) between the dichroic beam splitter 440 and thesteering mirror 430. In FIG. 4C, the steering mirror 430 has a firstorientation, which directs data modulated laser light 446 from the λ1laser light source 442 toward the direction 431. Simultaneously, thesteering mirror 430 directs incoming λ2 light 448 from direction 431toward λ2 detector 444. While in the first orientation, the opticalterminal 420 is configured to operate in the first mode and conduct fullduplex communication with a remote terminal located in direction 431.

FIG. 4D is a simplified block diagram of the optical communicationterminal 420 aligned for operation in a second mode of full duplexcommunication. In the second mode, the optical communication terminal420 transmits at λ2 and receives at λ1, and the steering mirror 430directs the data modulated light 429 to/from fixed optical components indirection 431 (toward the other terminal). In FIG. 4D, the steeringmirror 430 has a second orientation, which directs data modulated laserlight 456 from the λ2 laser light 452 source toward the direction 431.Simultaneously, the steering mirror 430 directs incoming λ1 light 454from direction 431 toward λ1 detector 450. While in the secondorientation, the optical terminal 420 is configured to operate in thesecond mode and conduct full duplex communication with a remote terminallocated in direction 431.

To allow the steering mirror 420 to align the outgoing and incominglight with the mode-specific emitter/detector pairs, and then changemodes solely by changing the alignment of the steering mirror 430, theoptical pathways within the optical terminal are angularly offset fromone another by a common amount. That is, within the terminal 430, theoptical pathway traversed by outgoing λ1 light between the λ1 laser 442and the steering mirror 430 (in the first mode) is angularly separatedfrom the optical pathway traversed by incoming λ1 light between the λ1detector 450 and the steering mirror 430 (in the second mode). Theangular separation allows the outgoing/incoming light to be aimedalternately at the λ1 laser 442 or the λ1 detector 450 via manipulationof the steering mirror 430, depending on the mode of operation.Similarly, the optical pathway traversed by outgoing λ2 light betweenthe λ2 laser 452 and the steering mirror 430 (in the second mode) isangularly separated from the optical pathway traversed by incoming λ2light between the λ2 detector 444 and the steering mirror 430 (in thefirst mode). The angular separation between the two λ1 optical pathwayscan be the same as the angular separation between the two λ2 opticalpathways.

By arranging the laser light sources 442, 452, the detectors 444, 450,and the dichroic beam splitter 440 to achieve a common angularseparation as described, the terminal 420 can change between modes ofoperation solely by moving the steering mirror 430, and activating thecorresponding light source and detector. As a result, the terminal 420can be dynamically reconfigured to switch modes of full duplexcommunication solely by moving the steering mirror, and the remainingoptical components in the terminal 420 can remain fixed, and alignmentcan be maintained. The arrangement thereby allows the terminal 420 toswitch modes of operation without tuning or adjusting any components inthe optical terminal other than the steering mirror 420.

In practice, the “first orientation” and “second orientation” of thesteering mirror 430 may be substantially fixed angular orientations fora given direction 431. But to communicate with a remote terminal in anarbitrary direction (within the field of view of terminal 420), theterminal 420 may be configured to orient the mirror through a range ofangles so as to direct outgoing and incoming light accordingly.Generally, for a given direction of optical free space propagation(i.e., a given lightpath) there are two different possible orientationsof the steering mirror 430: one which aligns the transmitted/receivedlight with a λ1 emitter and λ2 detector (for first mode communication),and another which aligns the transmitted/received light a λ2 emitter andλ1 detector (for second mode communication). The optical terminal cantherefore include a control system that can select and maintain anorientation of the steering mirror 430 based on the mode of full duplexcommunication and the direction of the lightpath.

In some cases, modes of operation for the individual opticalcommunication terminals in the network 400 can be assigned by a centralcontrol system (e.g., the control system 200 described in connectionwith FIG. 2). In other cases, modes of operation can be established onan ad hoc basis based on initial communications amongst the individualballoons in the network 400 to form the links that make up the meshnetwork.

4. Example Full Duplex Optical Communication Terminals

FIG. 5 is a diagram of an example optical communication terminal 500arranged to operate alternately in two different modes. The opticalcommunication terminal 500 may be similar in some respects to theoptical communication terminal 420 described above, and may beconfigured to be mounted to a payload of a high altitude platform, suchas a balloon in the networks described above. The terminal 500 includesa steering mirror 502, a dichroic beam splitter 518 that reflects λ1 andtransmits λ2, and thereby divides the light based on wavelength. Thediagram in FIG. 5 illustrates two distinct optical paths through theoptical terminal 500, which correspond to alternate operation in thefirst mode and the second mode. Dashed lines show the optical paths oftransmitted λ1 light and also received λ2 light, which are traversedduring operation in the first mode. Dotted lines show the optical pathsof transmitted λ2 light and also received λ1 light, which are traversedduring operation in the second mode. The distinct optical pathscorrespond to different orientations of the beam steering mirror 502,and are angularly separated by a small enough amount that they shareoptical components, such as focusing/collimating optics, filters, beamsplitters, and the like. However, the distinct paths are also angularlyseparated by a large enough amount that they direct light to/fromspatially separated locations within the optical terminal 500 to therebyenable the distinct modes of operation.

4a) Wavelength Differentiated Optical Paths

The features of the optical terminal 500 are described generally withreference to incoming light, although it is understood that light maypropagate in the reverse (outgoing) direction as well. Collimated beamsof light from another remote terminal propagate along a free spaceoptical pathway to the terminal 500 and are reflected by the steeringmirror 500, which directs the incoming light to relay optics 506. Therelay optics can include one or more lenses, reflectors, apertures, etc.that relay the collimated beams of light between the steering mirror 502and the remaining optics components within the terminal 500. The relayoptics 506 may scale the beam to another size, if desired, and output acollimated beam toward a beam splitter 508. The beam splitter 508diverts some of the incoming light toward an orientation feedback sensor514 for use in controlling the alignment of the steering mirror 502,which is described further below. The beam splitter 508 may, forexample, divert about 2% to 5% of the incoming light, with the remaininglight transmitted to a dichroic beam splitter 518.

The dichroic beam splitter 518 can be an optical element with awavelength-dependent transmission/reflection profile. In some cases, thedichroic beam splitter 518 may be implemented by a wavelength-dependentcoating applied to another optical element, such as a lens. The dichroicbeam splitter 518 may substantially reflect light having wavelength λ1while substantially transmitting light having wavelength λ2. Thedichroic beam splitter 518 therefore results in the incoming lighttraversing different optical paths, depending on wavelength. Lighthaving wavelength λ1 may enter a branch that is substantially transverseto the optical axis of the relay optics 506, and light having wavelengthλ2 may continue to a branch parallel to the optical axis of the relayoptics.

Along the λ1 branch, the light passes through a filter 520, which helpsblock light having other wavelengths. The filter 520 may transmit arange of wavelength that includes λ1. The filter 520 may alsosubstantially block λ2 light, which helps mitigate cross talk from lighton the other branch of the optical terminal 500. The filtered λ1 lightis then focused by focusing optics 522 to illuminate a detectionlocation 524. The focusing optics 522 can include a lens with a suitablefocal length to focus the collimated beam of λ1 light onto the detectionlocation 524 while the terminal operates in the second mode, as shown bythe dotted lines. The detection location 524 can be optically coupled toa λ1 detector 542 (e.g., an avalanche photodiode or the like) via afiber connection, for example. In some examples, the detection location524 may be implemented by an aperture in a ferrule that terminates afiber optic cable, which is connected to the λ1 detector 542.

In the reverse direction of light propagation, light from a λ1 laser 544can be fiber coupled to an emission location 526. As shown by the dashedlines, in the first mode, λ1 light emitted from the emission location526 propagates toward the focusing optics 522, which collimates theemitted light and directs the light back through the filter 520 to thedichroic beam splitter 518, where the λ1 light is reflected toward thesteering mirror 502 (via the relay optics 506) for transmission over thefree space optical path. The emission location 526 may be implemented byan aperture in a ferrule that terminals a fiber optic cable, which isconnected to the λ1 laser 544. In some cases, the detection location 524and emission location 526 may be implemented by a dual core fiberferrule, and the respective optical signals can then be conveyed viarespective cores in a fiber to the λ1 detector 542 and λ1 laser 544.

Referring again to incoming light, along the λ2 branch (from λ2 lightthat is transmitted through the dichroic beam splitter 518), the lightpasses through a filter 528, which helps block light having otherwavelengths. The filter 528 may transmit a range of wavelength thatincludes λ2. The filter 528 may also substantially block λ1 light, whichhelps mitigate cross talk from light on the other branch of the opticalterminal 500. The filtered λ2 light is then focused by focusing optics530 to illuminate a detection location 534. The focusing optics 530 caninclude a lens with a suitable focal length to focus the collimated beamof λ2 light onto the detection location 530 while the terminal 500operates in the first mode, as shown by the dashed lines. The detectionlocation 530 can be optically coupled to a λ2 detector 548 (e.g., anavalanche photodiode or the like) via a fiber connection, for example.In some examples, the detection location 534 may be implemented by anaperture in a ferrule that terminates a fiber optic cable, which isconnected to the λ2 detector 548.

In the reverse direction of light propagation, light from a λ2 laser 546can be fiber coupled to an emission location 532. As shown by the dottedlines, in the second mode, λ2 light emitted from the emission location532 propagates toward the focusing optics 530, which collimates theemitted light and directs the light back through the filter 528 to thedichroic beam splitter 518, where the λ2 light is transmitted toward thesteering mirror 502 (via the relay optics 506) for transmission over thefree space optical path. The emission location 532 may be implemented byan aperture in a ferrule that terminals a fiber optic cable, which isconnected to the λ2 laser 546. In some cases, the detection location 534and emission location 532 may be implemented by a dual core fiberferrule, and the respective optical signals can then be conveyed viarespective cores in a fiber to the λ2 detector 548 and λ2 laser 546.

4b) Orientation Feedback

As noted above, the optical terminal 500 also includes an orientationfeedback sensor 514, which detects a portion of incoming light that isdiverted by the beam splitter 508 after being reflected by the steeringmirror 502. The diverted light (which may be about 2% to 5% of the lightincident on beam splitter 508) passes through filter 510 (which may passboth λ1 and λ2 light and block light at other wavelengths), and is thenfocused by focusing optics 512 on the orientation feedback sensor 514.The orientation feedback sensor 514 can include a photo-sensitive array516, such as an array of photodiodes that generate electrical signalsbased on the pattern of light illuminating the array. In the first mode,λ2 light received from a remote terminal illuminates the photo-sensitivearray 516, as indicated by the dashed lines. In the second mode, λ1light received from the remote terminal illuminates the photo-sensitivearray 516, as indicated by the dotted lines.

The position on the photo-sensitive array 516 that is illuminateddepends on the angle of arrival of the incident light. Thus, theillumination pattern detected using the photo-sensitive array 516 can beused to determine the orientation of the steering mirror 502. Forexample, a centroid position of the light focused on the photo-sensitivearray 516 may be determined, and the orientation of the steering mirror502 may be determined based on the centroid position. Such adetermination may be made, for example, using a controller 550 thatreceives signals from the orientation feedback sensor 514 indicative ofthe measured illumination pattern. Based on the signals from theorientation feedback sensor 514, the controller 550 can generatecommands to the mirror positioning system 504 to thereby adjust theorientation of the steering mirror 502 and thereby align the incidentlight in a desired manner. Moreover, because the optical components inthe terminal 500 are fixed with respect to one another (e.g., by beingmounted to a common frame structure), different centroid positions ofthe light diverted to the photo-sensitive array 516 correspond todifferent alignments of the light that continues to the λ1 or λ2branches. In particular, there are two particular target locations onthe photo-sensitive array 516 where incident light corresponds to analignment in which the terminal 500 operates in the first mode or in thesecond mode.

For example, when the diverted λ1 light is focused on the locationmarked B, where the dotted lines converge on the photo-sensitive array516, the λ1 light that is not diverted by the beam splitter 508 isaligned to illuminate the detection location 524, and the λ2 lightemitted from the emission location 532 is simultaneously directed (viathe steering mirror 502) so as to propagate in the same direction fromwhich the λ1 light is received (i.e., toward the remote terminal). Thus,the target location B can be a location on the photo-sensitive array 516associated with operation of the terminal 500 in the second mode.Similarly, when the diverted λ2 light is focused on the location markedA, where the dashed lines converge on the photo-sensitive array 516, theλ2 light that is not diverted by the beam splitter 508 is aligned toilluminate the detection location 534, and the λ1 light emitted from theemission location 526 is simultaneously directed (via the steeringmirror 502) so as to propagate in the same direction from which the λ2light is received (i.e., toward the remote terminal). Thus, the targetlocation A can be a location on the photo-sensitive array 516 associatedwith operation of the terminal 500 in the first mode. The targetlocations A and B may be substantially fixed locations on thephoto-sensitive array 516 and may be determined based on a calibrationroutine. In some examples, the two target locations A and B can bepositions on a single photo-sensitive array (e.g., the photo-sensitivearray 516). Although in some examples, the orientation feedback sensor514 can include two distinct photo-sensitive arrays located at each ofthe target locations A and B.

4c) Controller

The controller 550 may be configured to use measurements of lightilluminating the photo-sensitive array 516 to adjust the orientation ofthe steering mirror 502. The controller 550 may cause the steeringmirror 502 to adjust such that the centroid position of lightilluminating the photo-sensitive array moves closer to one of the twotarget locations, depending on the mode of operation. For example, thecontroller 550 may obtain a measurement of an illumination pattern onthe photo-sensitive array 516, determine an adjustment to theorientation of the steering mirror 502 that would shift the illuminationpattern closer to a desired target location, and then instruct thepositioning system 504 accordingly. Moreover, the controller 550 mayoperate on an ongoing basis to adjust the steering mirror 502 andthereby track subtle changes in the direction of the incident light dueto relative movement of the remote terminal, for example.

In one example, the controller 550 may include a processing unit 552,data storage 556, and one or more input/output ports 554, which may becommunicatively linked together by a system bus, or one or more otherconnection mechanisms 562. The data storage 556 may include anon-transitory computer readable medium and includes position data 558and operating instructions 560. The instructions 560 may include, forexample, program logic that, when executed by the processing unit 552,cause the controller 550 to carry out the functions described herein.Thus, the instructions 560 may cause the controller 550 to determine anadjustment to the orientation of the steering mirror 502 based on datafrom the orientation feedback sensor 514, and to generate correspondinginstructions for the mirror positioning system 504. The position data558 may include stored indications of particular target locations on thephoto-sensitive array that correspond to the two different modes of fullduplex communication. The position data 558 may therefore be establishedduring a calibration routine, for example. The input/output ports 554function to receive data from the orientation feedback system 514 andalso to provide command instructions to the mirror positioning system504.

In addition, the controller 550 may, perhaps in coordination with otherentities, function to receive mode selection commands from a centralcontroller (or other entity) and to cause the terminal 500 to configureitself accordingly (e.g., by aligning with the corresponding targetlocation).

Further, the controller 550 may, perhaps in coordination with otherentities, function to operate the terminal to send and receive data. Thecontroller 550 and/or modem 540 may, while in the first mode, receiveoutput data for transmission over the free space optical link, and causethe λ1 laser 544 to emit light that is modulated in accordance with theoutput data. Simultaneously, the controller 550 and/or modem 540 mayextract input data based on the modulation of light detected with the λ2detector 548. Similarly, the controller 550 and/or modem 540 may, whilein the second mode, receive output data for transmission over the freespace optical link, and cause the λ2 laser 546 to emit light that ismodulated in accordance with the output data. Simultaneously, thecontroller 550 and/or modem 540 may extract input data based on themodulation of light detected with the λ1 detector 542.

While the terminal 500 is described generally in terms of opticalsignals at wavelengths λ1 and λ2, it is understood that the twowavelengths may have a range of different values. In some cases,wavelengths that are not readily absorbed within the atmosphere may beselected. For example, λ1 may be about 1540 nanometers, and λ2 may beabout 1560 nanometers. Of course, in another example λ1 may be 1560 andλ2 may be 1540, such that the greater of the two wavelengths is divertedtransverse, along the λ1 branch of the terminal 500. However, many otherwavelengths in the ultraviolet, visible, and near infrared spectrum mayalso be selected. In addition, for given values of λ1 and λ2, thevarious filters 510, 520, 528, lasers 542, 546, detectors 544, 548, andthe dichroic beam splitter 518 may be selected to achieve the wavelengthspecific behaviors described herein (including selection of lasingmedia, photodiodes, coatings, etc.). Moreover, the photo-sensitive array516 of the orientation feedback sensor 514 may be implemented with atechnology suitable for detection of the received wavelength. As oneexample, a sensor array formed from photodiodes including Indium GalliumArsenide (InGaAs) elements may be suitable for detection of light froman Erbium doped solid state laser diode (e.g., in a band around 1550nanometers). Many other examples are possible, including wavelengths ina band around 1000 nanometers implemented with laser diodes havinglasing media that include yttrium aluminum garnet (YAG) doped with avariety of materials.

4d) Alternative Orientation Feedback Sensor

FIG. 6 is a diagram of another example optical communication terminal600. The terminal 600 is similar in some respects to the terminal 500described above in connection with FIG. 5, and may be mounted to apayload of a high altitude platform, such as a balloon, in a network toprovide free space optical communication links between differentballoons. However, the terminal 600 includes an alternative arrangementto the orientation feedback sensor described above. The terminal 600includes a coarse position sensor 624 and a fine position sensor 614,which each receive a fraction of the light that is diverted by the beamsplitter 508 and passes through the filter 510. In the exampleconfiguration illustrated in FIG. 6, a second beam splitter 610 dividesthe beam of incoming light from the beam splitter 508 and reflects sometoward the fine position sensor 614, and transmits some toward thecoarse position sensor 624. The beam splitter 610 may divide the lightsuch that about half goes each direction, although other possibilitiesmay be employed (e.g., 40/60, 30/70, etc.). In some examples, theterminal 600 may be implemented with the fine position sensor 614 andthe coarse position sensor 624 situated in a variety of differentpositions along the optical pathway of the terminal 600 such that eachsensor receives at least some light reflected by the steering mirror502. As such, the fine position sensor 614 and coarse position sensor624 can be used to provide feedback on the orientation of the steeringmirror 502, and thus the mode of operation of the terminal 600.

Focusing optics 622 focus the light on a photo-sensitive array 626 ofthe coarse position sensor 624 and focusing optics 612 focus the lighton a photo-sensitive array 616 of the fine position sensor 614. Each ofthe photo-sensitive arrays 616, 626 have target locations associatedwith alignment for operation in the first mode or in the second mode.The coarse position sensor 624 is used to provide coarse orientationfeedback to the steering mirror 502 and the fine position sensor 614 isused to provide fine tuning of the orientation. The angles of incidentlight that can be detected with the coarse position sensor 624 maytherefore span a greater range, but with coarser granularity, than theangles of incident light that can be detected by the fine positionsensor 614. Generally, the angular span that can be detected, and theextent to which particular angles can be resolved, with either of theposition sensors 614, 624 depends both on the respective focusing optics622, 612 and the size and resolution of the respective photo-sensitivearrays 616, 626. For example, both photo-sensitive arrays 616, 626 maybe substantially similar, but the focusing optic 622 may have arelatively short focal length, which allows the photo-sensitive array614 to be positioned closer to the optic 622, and a greater angularrange of incident light to illuminate the photo-sensitive array 626. Bycontrast, the focusing optic 612 may have a relatively long focallength, which allows the photo-sensitive array 616 to be positionedfurther away, and a lesser angular range of incident light to illuminatethe photo-sensitive array 616, but with greater resolution betweenangles.

In some examples, the coarse position sensor 624 may be used primarilyto align the steering mirror 502 with sufficient accuracy to get theincident light to illuminate the photo-sensitive array 616 of the fineposition sensor 614. In some examples, moreover, the terminal 600 mayalso include an additional fine-steering mirror in the optical pathwayof the terminal that receives light reflected by a primarycoarse-steering mirror (e.g., the steering mirror 502). In such anarrangement, the coarse-steering mirror can receive orientation feedbackfrom a coarse position sensor. The coarse position sensor can then beused to orient the coarse-steering mirror such an incident light isreflected by the fine-steering mirror. The fine-steering mirror can thenreceive orientation feedback from a fine position sensor, which issituated to receive light reflected by the fine-steering mirror.

In another example, the two photo-sensitive arrays 616, 626 may havedifferent configurations. For instance, the fine position sensor array616 may be a quad cell detector that detects a position of an incidentbeam relative to an intersection of four pixel detectors or anotherposition-sensing detector. The coarse position sensor array 626 may be aphoto-sensitive array with a larger field of view, such as atwo-dimensional array of pixel detectors similar to those employed inimaging applications. Moreover, the fine position sensor array 616(e.g., quad cell detector) may be operated at a relatively higherread-out rate than the coarse position sensor array 626, and therebyallow for more frequent feedback to adjust the orientation of thesteering mirror 502.

Feedback from both position sensors 624, 614 can be provided to thecontroller 650 which determines an adjustment to the orientation of thesteering mirror 502 so as to align the position of the measured lightwith a mode-specific target location (e.g., the locations labelled withA and B in FIG. 6), and instructs the mirror positions system 504accordingly. In some examples, when forming an initial link, thecontroller 600 may initially check whether a centroid position ofincoming light can be detected on the fine position sensor 614. If not,such as if the initial alignment is offset by too much to reach the fineposition sensor 614, the controller 600 may begin adjusting theorientation of the steering mirror 502 based on information from thecoarse position sensor 624 and then transition to using the fineposition sensor 614 once the detected centroid position is near thecoarse position target location. In some cases, the coarse positionsensor 624 and/or fine position sensor 614 may include multiple distinctphoto-sensitive detectors, with one detector situated at the respectivemode-specific target locations, similar to the orientation feedbacksensor 514 described above. For example, the fine position sensor array616 may include two quad cell detectors, one at target location A, andone at target location B. Further still, the controller 650 may beconfigured to use a combination of measurements from the two positionsensors 614, 624 simultaneously.

4e) Alignment Laser

FIG. 7 is a diagram of another example optical communication terminal700. The terminal 700 is similar in some respects to the terminals 500described above in connection with FIG. 5, and may be mounted to apayload of a high altitude platform, such as a balloon, in a network toprovide free space optical communication links between differentballoons. However, the terminal 700 includes an alignment laser lightsource 728 that emits light at a third wavelength λ3 that is differentfrom both λ1 and λ2. Another dichroic element 710 substantiallytransmits λ1 light and λ2 light, and substantially reflects λ3 light.The dichroic element 710 is positioned to receive incoming light fromthe relay optics 506, and thereby divert incoming λ3 light along a λ3optical path distinct from the λ1 and λ2 branches. The diverted λ3 lightpasses through a filter 712 that selectively transmits λ3 light, andthen a beam splitter 720 allows some of the incoming λ3 light to passtoward an orientation feedback sensor 716. The beam splitter 720 directslight from the λ3 laser light source 728 for transmission to the remoteterminal, via the dichroic element 710, relay optics 506, and steeringmirror 502.

The alignment laser 728 is optically coupled to an emission location 726via a fiber. An optic 724 collimates λ3 light from the emission location726 and directs the light toward the beam splitter 720. The beamsplitter 720 can be an optic element that partially reflects light at λ3and partially transmits light at λ3. As noted above, the beam splitter720 diverts at least some of the λ3 light to be transmitted from theterminal 700, and also allows some incoming λ3 light (from the remoteterminal) to pass through to the orientation feedback sensor 716.

After entering the λ3 branch, the incoming λ3 light passes through afilter 712 and is focused on a photo-sensitive array 718 of theorientation feedback sensor 716 by focusing optics 714. The orientationfeedback sensor 716 can be similar to the orientation feedback sensor514 described in connection with FIG. 5, except that the photo-sensitivearray 718 is configured to detect light at λ3 rather than λ1 and λ2. Theorientation feedback sensor 716 provides signals to the controller 750indicative of measured λ3 light that illuminates the photo-sensitivearray 718. And the controller 750 can operate the mirror positioningsystem 504 to adjust the orientation of the steering mirror 502 to causethe centroid position of the illuminating light to become aligned with amode-specific target location (e.g., the locations labelled A and B inFIG. 7).

The dichroic element 710 is configured to both divert at least someincoming λ3 light toward the orientation feedback sensor 716 yet stilltransmit at least some outgoing λ3 light from the λ3 laser, and also tosubstantially transmit all light at λ1 and λ2. The dichroic element 710may therefore be implemented by an optic partially coated with a layerthat selectively reflects λ3, which divides the λ3 light between areflected portion and a transmitted portion. Moreover, while the twodichroic elements 710, 720 are illustrated as items which reflect λ3 (atleast partially) and transmit λ1 and λ2, some embodiments may includedichroic elements that transmit λ3 and reflect λ1 and λ2, in which casethe λ1 and λ2 optical paths may be directed transverse to the opticalaxis of the relay optics 506. Similarly, either the alignment laseremission location 726 or the orientation feedback sensor 716 may belocated parallel to the optical axis of the relay optics. Generally,however dichroic elements 710, 720 are arranged to cause λ3 light totraverse a different optical path through the terminal 700, and yetstill align with the transmitted/received light at λ1 and λ2 reflectedby the steering mirror 502.

Using a separate alignment laser having a wavelength (e.g., λ3) thatdiffers from the communication wavelengths (e.g., λ1 and λ2) may allowsome benefits. The power of the λ3 light illuminating the orientationfeedback sensor 716 does not come at the expense of power in thecommunication signals, as in the terminals 500 and 600. Separating thealignment sensor at a non-communication wavelength allows the signal tonoise of the λ3 light provided to the orientation feedback sensor 716 tobe substantially independent of the signal to noise of the communicationsignals.

In addition, the photo-sensitive array 718 can be implemented to providephotodiodes that are sensitive to light at λ3, independent of thecommunication wavelengths. For example, even with communicationwavelengths in a band near 1550 nanometers, the alignment laser may beat 904 nanometers, and the photo-sensitive array 718 may be implementedusing a complementary metal oxide semiconductor (CMOS) array ofphotodiodes, rather than InGaAs.

Generally, the various features of the optical communication terminalsdescribed herein may be combined in a variety of different ways. Forinstance, an optical communication terminal may be implemented toinclude both the alternative orientation feedback sensor arrangement ofthe terminal 600 described in connection with FIG. 6 in combination witha separate alignment laser described in connection with FIG. 7.

In addition, while not specifically illustrated in the diagrams of FIGS.5-7, each of the optical terminals 500, 600, 700 also includes a frame,housing, or other structural feature that the various optical componentsare mounted to. The frame structure can maintain the relative spacing,orientation, and/or positions of each the optical components (e.g., thelenses, dichroic elements, reflectors, the emission locations, thedetection locations, etc.) such that the wavelength-specific paths ofthe first mode and the second mode remain in alignment with respect toone another.

4f) Fiber Optic Ferrule

FIG. 8A is a diagram of an example optical communication ferrule.Ferrule 800 is shown in a view looking at the ferrule through the end ofthe ferrule connector. The ferrule 800 may be used to couple light intoand out of the systems disclosed herein. For example, detection location524, emission location 526, emission location 532, and detectionlocation 534 of FIG. 5 may each be located at a tip of a fiber opticcable located within a ferrule 800. The ferrule 800 may be used as theconnection point between the signal receiving units disclosed herein andthe further processing and signal creation systems.

The ferrules shown in FIGS. 8A and 8B may be used in connectors that caninject signals into and remove signals from the optical communicationunits of the present disclosure. The system may have at least twoferrules similar to ferrule 800. The two ferrules may be interchangeablewith each other. By having the ferrules be interchangeable, theconnections to the optical communication system may be made in a morecost effective manner. Additionally, interchangeable ferrules may alloweasier repairs on the optical communication system. Further, the each ofthe ferrules may be configured to couple to the optical communicationsystem so that each ferrule has fibers configured to communicate withone of the two frequencies of the communication system.

The ferrule 800 may include an outer sleeve 802. The outer sleeve 802may be configured to couple the ferrule 800 to an optical communicationterminal. In some examples, the outer sleeve 802 may include a screwmechanism. The screw mechanism may enable the outer sleeve 802 tosecurely couple the ferrule 800 to the optical terminal. The outersleeve 802 may use other coupling means to couple the ferrule 800 to theoptical terminal. The means of coupling the ferrule 800 to the opticalterminal may take many different forms depending on the respectiveembodiment. In some examples, the outer sleeve 802 of the ferrule 800may be similar to an FC connector as defined by the standardEIA/TIA-604-4.

The ferrule 800 may also include an internal sleeve 804. In variousembodiments, the internal sleeve 804 may take many different forms. Insome examples, the internal sleeve 804 may be made of a rigid orsemi-rigid material. The internal sleeve 804 may function as aprotective sleeve for the two fibers 806 and 808 that are present withinthe ferrule 800. Further, the internal sleeve 804 may be metallic andprovide some electromagnetic shielding of the two fibers 806 and 808. Asshown in FIG. 8A, the internal sleeve 804 may have a geometry that isincludes a flat portion. The flat portion of the internal sleeve 804 mayenable an ease of alignment when the ferrule 800 is used to couple lightinto and out of the systems. In some further examples, the internalsleeve 804 may include alignment features other than the flat portion.For example, alignment features may include a notch, alignment tab, orother alignment component. Further, in some examples, the alignmentfeatures may be found on both, or one of, the outer sleeve 802 and/orthe internal sleeve 804.

Within the internal sleeve 804 are two the two fibers 806 and 808 thatform the fiber optic communication pathway. The two fibers 806 and 808may be a transmission fiber 806 and a reception fiber 808. In someexamples, the transmission fiber 806 may be a single-mode fiber and thereception fiber 808 may be a multi-mode fiber. A single-mode fiber is afiber that allows light propagation in only a single mode (e.g. atransverse mode) across the frequencies of light carried by the fiber. Asingle-mode fiber may be useful in communications because thesingle-mode fiber may provide a light output with a more narrow andfocused beam of light as compared to multi-mode fibers. The receptionfiber 808 may be a multi-mode fiber. A multi-mode fiber is a fiber thatallows light propagation in multiple modes (e.g. not only the transversemode) across the frequencies of light carried by the fiber. Themulti-mode fiber may provide the benefit that a multi-mode fiber may beable to receive light from a wider angle as compared to a single-modefiber. Therefore, the two fibers 806 and 808 of the ferrule 800 may beconfigured so that a narrow-beamed single-mode fiber 806 is configuredfor signal transmission and a wide-beamed multi-mode fiber 808 isconfigured for signal reception.

The alignment of the two fibers 806 and 808 within the internal sleeve804 is one example alignment of the two fibers 806 and 808. Thealignment of the two fibers 806 and 808 may be determined based on thedesign of the optical communication system. In practice, when theoptical communication system is designed, a location of the detectionlocation 524, emission location 526, emission location 532, anddetection location 534 of FIG. 5 may be part of the design criteria. Thetwo fibers 806 and 808 may each be located at a tip of a fiber opticcable located within a ferrule 800 at the corresponding emission anddetection location. The optical communication system may be furtherdesigned so that detection location 524 and emission location 526 havethe same relative spacing as emission location 532 and detectionlocation 534. By having the same relative spacing between detection andemission locations, a single ferrule 800 may be able to both couplesignals associated with both frequencies of light with which the opticalcommunication system may operate.

FIG. 8B is a diagram of another example optical communication ferruleand cables. The ferrule and cables 850 of FIG. 8B may show morecomponents compared to FIG. 8A. The ferrule 852 of the ferrule andcables 850 may be the same or similar to ferrule 800 of FIG. 8A.

The ferrule 852 may have two cables (e.g. fiber optic cables) 854A and854B coupled to the ferrule. The cables 854A and 854B may be configuredto provide the signals to a and from the tips of the two fibers (806 and808 of FIG. 8A) to the signal generation and signal reception equipment.For example, cable 854A may be a single-mode fiber that provides thesignal transmitted at the fiber tip of the optical communication system.Thus, the fiber 806 of FIG. 8A may be the tip of the fiber containedwithin cable 854A. Cable 854A may be further coupled to a connector865A. The connector 856A may connect a laser source that creates thetransmission signal to the fiber within cable 854A. Cable 854B may be amulti-mode fiber that communicates the signal received at the fiber tipof the optical communication system. Thus, the fiber 808 of FIG. 8A maybe the tip of the fiber contained within cable 854B. Cable 854B may befurther coupled to a connector 865B. The connector 856B may connect areceiver that can decode the received signal that propagated through thefiber within cable 854B.

The ferrule and cables 850 of FIG. 8B may be used with both outputs ofthe optical communication system. As discussed above with respect toferrule 800, the ferrule and cables 850 may be designed to beinterchangeable. That is, the optical communication system may have thesame size and shape connectors that operate with the componentsconfigured to communicate with the first frequency as those configuredto operate with the second frequency. Thus, the ferrule and cables 850are designed to be able to connect a laser source and a signal receiveroperating at a given frequency with the respective emission location anddetection location of the optical communication system.

5. Example Operation

FIG. 9 is a flowchart of an example process 900 for operating an opticalcommunication terminal according to an example embodiment. The process900 illustrated in FIG. 9 may be implemented by any of the opticalcommunication terminals described herein alone or in combination withhardware and/or software implemented functional modules, such ascontrollers situated aboard a balloon or at a ground station. At block902, the optical communication terminal is configured for making adetermination to conduct communication in one of two modes. Thedetermination may be made based on either an operating mode of a remoteterminal with which the optical communication system wishes tocommunicate or by the optical communication system determining how tocommunicate with the remote terminal. In some examples, the system maydefault to operating in the first mode.

The determination may be made on the basis of instructions from acentral controller or another balloon that instruct the opticalcommunication terminal to initiate an optical link with another balloon.Such instructions may include information for each balloon thatspecifies the approximate coordinates of the other balloon (e.g., GPScoordinates) and the mode of operation of each balloon. Thedetermination may also be made based on other factors, including adetermination made solely by the optical communication terminal. Forexample, the optical communication terminal may receive broadcastinformation regarding the relative position(s) of other nearby terminals(perhaps via a radio link), and may then attempt to establish an opticallink with a particular one of the terminals by searching for an opticalsignal to use for alignment (and also transmitting an optical signal touse for alignment). The terminal can then align itself to conductcommunication in one mode, and, if not successful after some period, mayswitch modes. The optical mesh network may thereby configure itself toestablish complementary modes of operation to provide full duplexcommunication in an organic manner that is not entirely planned by acentral controller.

At block 904, the optical communication terminal is configured tooperate in the first mode by orienting its steering mirror so as todirect light of a first frequency emitted from a transmission fiber of afirst ferrule toward a remote terminal while also directing incidentlight of a second wavelength received from the remote terminal toward areception fiber of the second ferrule. For example, the opticalcommunication terminal may include an orientation feedback sensor thatis illuminated by a portion of the incident light of the secondwavelength (or light at another wavelength used for alignment), andprovides feedback to a positioning system to orient the mirror so as toalign the light illuminating the orientation feedback sensor with atarget location.

At block 906, the optical communication terminal can conduct full duplexcommunication in the first mode by transmitting data modulated light ofa first frequency, and receiving data modulated light of a secondfrequency. For example, a modem may be used to both encode output datainto the transmitted light by instructing the laser to emit light of thefirst frequency with a particular modulation pattern that corresponds tothe output data. The modem may also extract (decode) data from thereceived light by detecting a modulation pattern of the received light,and identifying input data that corresponds to the modulation pattern.

At block 908, the optical communication terminal is configured tooperate in the second mode by orienting its steering mirror so as todirect light of a second frequency emitted from a transmission fiber ofa second ferrule toward a remote terminal while also directing incidentlight of a first wavelength received from the remote terminal toward areception fiber of the first ferrule. For example, the opticalcommunication terminal may include an orientation feedback sensor thatis illuminated by a portion of the incident light of the firstwavelength (or light at another wavelength used for alignment), andprovides feedback to a positioning system to orient the mirror so as toalign the light illuminating the orientation feedback sensor with atarget location.

At block 910, the optical communication terminal can conduct full duplexcommunication in the second mode by transmitting data modulated light ofthe second frequency, and receiving data modulated light of the firstfrequency. For example, a modem may be used to both encode output datainto the transmitted light by instructing the laser to emit light of thesecond frequency with a particular modulation pattern that correspondsto the output data. The modem may also extract (decode) data from thereceived light by detecting a modulation pattern of the received light,and identifying input data that corresponds to the modulation pattern.

In some embodiments, the disclosed methods may be implemented ascomputer program instructions encoded on a non-transitorycomputer-readable storage media in a machine-readable format, or onother non-transitory media or articles of manufacture. FIG. 10 is aschematic illustrating a conceptual partial view of an example computerprogram product that includes a computer program for executing acomputer process on a computing device, arranged according to at leastsome embodiments presented herein.

In one embodiment, the example computer program product 1000 is providedusing a signal bearing medium 1002. The signal bearing medium 1002 mayinclude one or more programming instructions 1004 that, when executed byone or more processors may provide functionality or portions of thefunctionality described above with respect to FIGS. 1-9. In someexamples, the signal bearing medium 1002 may encompass a non-transitorycomputer-readable medium 1006, such as, but not limited to, a hard diskdrive, a Compact Disc (CD), a Digital Video Disk (DVD), a digital tape,memory, etc. In some implementations, the signal bearing medium 1002 mayencompass a computer recordable medium 708, such as, but not limited to,memory, read/write (R/W) CDs, R/W DVDs, etc. In some implementations,the signal bearing medium 1002 may encompass a communications medium1010, such as, but not limited to, a digital and/or an analogcommunication medium (e.g., a fiber optic cable, a waveguide, a wiredcommunications link, a wireless communication link, etc.). Thus, forexample, the signal bearing medium 1002 may be conveyed by a wirelessform of the communications medium 1010.

The one or more programming instructions 1004 may be, for example,computer executable and/or logic implemented instructions. In someexamples, a computing device such as the computer system 312 of FIG. 3may be configured to provide various operations, functions, or actionsin response to the programming instructions 1004 conveyed to thecomputer system 312 by one or more of the computer readable medium 1006,the computer recordable medium 1008, and/or the communications medium1010.

The non-transitory computer readable medium could also be distributedamong multiple data storage elements, which could be remotely locatedfrom each other. The computing device that executes some or all of thestored instructions could be a device, such as the balloon 300 shown anddescribed in reference to FIG. 3 or another high altitude platform.Alternatively, the computing device that executes some or all of thestored instructions could be another computing device, such as a serverlocated in a ground station.

The above detailed description describes various features and functionsof the disclosed systems, devices, and methods with reference to theaccompanying figures. While various aspects and embodiments have beendisclosed herein, other aspects and embodiments will be apparent tothose skilled in the art. The various aspects and embodiments disclosedherein are for purposes of illustration and are not intended to belimiting, with the true scope being indicated by the following claims.

What is claimed is:
 1. An optical communication terminal comprising: abeam splitter configured to transmit light of a first wavelength andreflect light of a second wavelength; at least two ferrules comprising afirst ferrule and a second ferrule, wherein each ferrule comprises aconnector, a transmission fiber, and a reception fiber, and wherein thefirst ferrule is configured to couple light transmittable by the beamsplitter to at least one fiber and the second ferrule is configured tocouple light reflectable by the beam splitter to at least one fiber,wherein each transmission fiber is a single-mode fiber that allows lightpropagation in only a single mode and each reception fiber is multi-modefiber is that allows light propagation in multiple modes, and wherein asize of the first ferrule is the same as a size of the second ferrule;and a steering mirror, wherein the steering mirror is positionable in atleast a first orientation and a second orientation; wherein the steeringmirror and the beam splitter are arranged such that, while the steeringmirror has a first orientation, (i) light of the first wavelength thatis emitted from the transmission fiber of a first ferrule is directedfor transmission toward a remote terminal, and (ii) light of the secondwavelength that is received from the remote terminal is directed towardthe reception fiber of a second ferrule, and wherein the steering mirrorand the beam splitter are further arranged such that, while the steeringmirror has a second orientation, (i) light of the second wavelength thatis emitted from the transmission fiber of the second ferrule is directedfor transmission toward a remote terminal, and (ii) light of the firstwavelength that is received from the remote terminal is directed towardthe reception fiber of the first ferrule.
 2. The optical communicationterminal of claim 1, wherein each ferrule is further configured: thetransmission fiber configured to operate a single-mode fiber; and thereception fiber configured to operate as a multi-mode fiber.
 3. Theoptical communication terminal of claim 1, further comprising: acontroller configured to (i) make a determination to switch betweenmodes of full duplex communication, and (ii) responsive to making thedetermination, cause the orientation of the steering mirror to changebetween the first orientation and the second orientation.
 4. The opticalcommunication terminal of claim 1, wherein the respective connector ofeach ferrule is the same sized connector as each other respectiveconnector.
 5. The optical communication terminal of claim 1, whereineach ferrule is further configured interoperable with each otherferrule.
 6. The optical communication terminal of claim 5, wherein eachferrule is further configured having the transmission fiber having afirst fiber location and the reception fiber having a second fiberlocation, wherein the respective fiber location is common within eachferrule.
 7. The optical communication terminal of claim 1, wherein eachferrule is further configured having an alignment element configured toalign the fibers of the respective ferrule.
 8. A high altitude platformcomprising: an envelope; a payload configured to be suspended from theenvelope; and an optical communication terminal mounted to the payload,the optical communication terminal comprising: (i) a beam splitterconfigured to transmit light of a first wavelength and to reflect lightof a second wavelength; (ii) at least two ferrules comprising a firstferrule and a second ferrule; and (iii) a steering mirror, wherein eachferrule comprises a connector, a transmission fiber, and a receptionfiber, and wherein a first ferrule is configured to couple lighttransmittable by the beam splitter to at least one fiber and a secondferrule is configured to couple light reflectable by the beam splitterto at least one fiber, wherein each transmission fiber is a single-modefiber that allows light propagation in only a single mode and eachreception fiber is multi-mode fiber is that allows light propagation inmultiple modes, and wherein a size of the first ferrule is the same as asize of the second ferrule; and wherein the steering mirror ispositionable in at least a first orientation and a second orientation;wherein the steering mirror and the beam splitter are arranged suchthat, while the steering mirror has a first orientation, (i) light ofthe first wavelength that is emitted from the transmission fiber of afirst ferrule is directed for transmission toward a remote terminal, and(ii) light of the second wavelength that is received from the remoteterminal is directed toward the reception fiber of a second ferrule, andwherein the steering mirror and the beam splitter are further arrangedsuch that, while the steering mirror has a second orientation, (i) lightof the second wavelength that is emitted from the transmission fiber ofthe second ferrule is directed for transmission toward a remoteterminal, and (ii) light of the first wavelength that is received fromthe remote terminal is directed toward the reception fiber of the firstferrule.
 9. The high altitude platform of claim 8, wherein each ferruleis further configured: the transmission fiber configured to operate asingle-mode fiber; and the reception fiber configured to operate as amulti-mode fiber.
 10. The high altitude platform of claim 8, furthercomprising: a controller configured to (i) make a determination toswitch between modes of full duplex communication, and (ii) responsiveto making the determination, cause the orientation of the steeringmirror to change between the first orientation and the secondorientation.
 11. The high altitude platform of claim 8, wherein therespective connector of each ferrule is the same sized connector as eachother respective connector.
 12. The high altitude platform of claim 8,wherein each ferrule is further configured interoperable with each otherferrule.
 13. The high altitude platform of claim 12, wherein eachferrule is further configured having the transmission fiber having afirst fiber location and the reception fiber having a second fiberlocation, wherein the respective fiber location is common within eachferrule.
 14. The high altitude platform of claim 8, wherein each ferruleis further configured having an alignment element configured to alignthe fibers of the respective ferrule.
 15. A method comprising: making adetermination to conduct full duplex communication in one of two modes;responsive to making the determination to conducting full duplexcommunication in a first mode: orienting a steering mirror so as to: (i)direct light of a first wavelength that is emitted from a transmissionfiber of a first ferrule toward a remote terminal, and (ii) directincident light of a second wavelength received from the remote terminaltoward a reception fiber of a second ferrule; conducting full duplexcommunication in a first mode by: (i) emitting light of the firstwavelength from transmission fiber of the first ferrule, modulated basedon output data, and (ii) receiving light of the second wavelength by thereception fiber of the second ferrule; responsive to making thedetermination to conducting full duplex communication in a second mode:orienting the steering mirror so as to: (i) direct light of the secondwavelength emitted from a transmission fiber of the second ferruletoward the remote terminal, and (ii) direct incident light of the firstwavelength received from the remote terminal toward a reception fiber ofthe first ferrule; and conducting full duplex communication in thesecond mode by: (i) emitting light of the second wavelength fromtransmission fiber of the second ferrule, modulated based on outputdata, and (ii) receiving light of the first wavelength by the receptionfiber of the first ferrule; wherein each transmission fiber is asingle-mode fiber that allows light propagation in only a single modeand each reception fiber is multi-mode fiber is that allows lightpropagation in multiple modes, and wherein a size of the first ferruleis the same as a size of the second ferrule.
 16. The method of claim 15,further comprising: transmitting the first wavelength of light towardthe first ferrule with a beam splitter; and reflecting a secondwavelength of light toward the second ferrule with the beam splitter.17. The method of claim 15, wherein light is emitted from a first fiberlocation of the ferrule and light is received by a second location ofthe ferrule.
 18. The method of claim 17, wherein light is emitted fromthe first fiber location is emitted in a single mode and light isreceived by a second location is received in multiple modes.
 19. Themethod of claim 15, wherein the first mode is a default mode configuredto operate in absence of a determination.
 20. The method of claim 15,further comprising extracting input data based on light of at least oneof the first wavelength and the second wavelength.